1. Field
The present disclosure relates to digital calibration of transmitters and receivers of quadrature modulated signals, implemented using an internally switched signal path. Keywords include algorithm, CORDIC, calibration, I/Q imbalance, local oscillator (LO), transceiver, transmitter (TX) and receiver (RX).
2. Background
Quadrature modulation is fundamental in nearly all wireless transceivers for its high spectral efficiency, but mismatches of components and physical layout generally leads to gain and phase mismatches between the I and Q quadrature signals. The resultant image effect can significantly degrade the error vector magnitude (EVM), especially for very dense signal constellations such as 16-QAM and 64-QAM. Since I/Q imbalances can happen in the receiver (RX), transmitter (TX) and local oscillator (LO), the desired calibration algorithm should be able to correct the I/Q imbalances with minimum add-on hardware for better cost and area efficiencies.
A number of techniques have been reported to deal with the I/Q imbalance problem. Examples are the loop-back detector with a recursive algorithm, two-dimensional iterative search algorithm, and blind estimation algorithm. The first two are demanding in hardware and computation, inducing a long calibration time and are power and area hungry in their implementation. For the third, although no training sequence is entailed, the estimation process still involves heavy computation (e.g., inverse matrix) and can suffer from the instability issue in the recursive loop.